This invention relates to batteries, and more particularly to a plate making process for lead acid batteries.
Lead acid batteries are the oldest and best-known energy devices in automobile applications. A common process to manufacture flat pasted plate lead acid batteries is shown schematically in FIG. 1. Pure lead 10 is converted in step 20 to a 70-80% oxidized lead powder (lead oxide or leady oxide) in a Barton pot or a ball mill with a range of grain size distribution. For the positive paste, the mixing step 30 includes placing the dry lead oxide powder from step 20 in a positive mixing machine, such as a 3000 pound paste mixer, and mixing it with water 40 and H2SO4 50 under constant stirring and at an elevated temperature. For the negative paste, mixing step 60 includes placing the dry lead oxide powder from step 20 in a negative mixing machine, such as a 3000 pound paste mixer, and mixing it with water 40, H2SO4 50 and an expander 70 under constant stirring at ambient temperature. The pastes formed from mixing steps 30 and 60, depending on the ratio of starting materials, the rate of mixing and the temperature, contain mixtures of the initial powders, lead sulfate, and basic lead sulfates such as PbOPbSO4 (monobasic lead sulfate), 3PbOPbSO4 H2O (tribasic lead sulfate), and 4PbOPbSO4 (tetrabasic lead sulfate).
After a period of mixing, the pasting step 80 includes pressing the respective pastes on the expanded grids by a specially designed machine to prepare the positive and negative plates. To prevent sticking of the plates, the positive and negative plates are surface dried in an oven prior to stacking them on the skids, as indicated at steps 90,100 respectively. To improve the active material/grid contact and the mechanical strength of the active material, the skids with positive plates from step 90 are subjected to a steaming and curing process 110, which includes transporting the positive plates to a steam chamber for several hours and then to a curing room for about 3-4 days. During steaming and curing 110, further reaction of the ingredients occurs, resulting in a different ratio of the lead oxides, sulfate and basic lead sulfates. The resulting cured material is a precursor to lead dioxide, which forms the active material in the plates.
After curing is complete, the plates from steps 115 and 110 are transported to assembly 120 and the battery formed. The formation step 130 includes electrochemically oxidizing the precursor material for the positive electrode to lead dioxide and for the negative electrode to sponge lead, typically by adding sulfuric acid into the assembled cells. The finishing step, also 130, includes dumping the forming acid, refilling the batteries with the shipping acid, and sealing the batteries with a final cover. The whole process of making a single battery may take at least 6-7 days. In batch-and-queue production, the process more commonly takes 3-4 weeks.
During the plate drying process, both the cross-section of the pores and the volume of the paste are decreased, leading to paste shrinkage. In some cases, the shrinkage of the paste is so extensive that cracks may occur, or the paste may even become detached from the grid. The cracks disrupt the electrical path in the plate and hinder the formation of the active materials. To eliminate paste cracking, it is critical to employ adequate drying conditions.
During the steaming and curing processes, several chemical reactions occur, including oxidation of residual lead, recrystallization of basic lead sulfate, paste drying and grid corrosion. The best bonding strength between the crystals of the paste, i.e. cohesion strength, and between the paste and the grid, i.e. adhesion strength, is obtained at the end of curing. Several factors, such as moisture content in the paste, structure of the active material crystals, and structure of the corrosion layer may affect the cohesion and adhesion strength of the plates. Due to long production time and stacking arrangement of the plates, it has become a challenge to produce crack-free strong plates with well-controlled quality, such as consistent paste density, porosity, crystal morphology and amount of active material pasted.
Some efforts at improving lead acid batteries have focused upon the precursor material. For example, U.S. Pat. No. 5,660,600 is designed to optimize the size and structure of tetrabasic lead sulfate crystals, which have a large impact upon the formation of the active material in the positive plate and the mechanical strength and cycling life of the positive plates. Tetrabasic lead sulfate crystallizes as large elongated prismatic (needle shape) crystals, but their formation is inefficient and their utilization (capacity per gram of active material) is lower than other oxides. In U.S. Pat. No. 5,660,600, reaction temperature and curing temperature are controlled to produce a uniform prismatic size of tetrabasic lead sulfate crystals purportedly having average width dimensions in the range of 1-2 xcexcm, thereby allowing rapid conversion to lead dioxide. In practice, however, the teachings of U.S. Pat. No. 5,660,600 result in a paste that is far too brittle to be useful.
Another approach at improving lead acid batteries is to eliminate the steaming and curing steps to provide a process that is less costly and time-consuming than the traditional process depicted in FIG. 1. To make cureless plates that have similar or better quality than traditional cured plates for lead acid batteries is an extremely challenging endeavor. One recently developed method for making cureless plates, disclosed in copending U.S. Pat. application Ser. No. 09/304,877, can be characterized in that the paste mixing, reacting and crystal forming steps occur in an extrusion or a high-shear continuous processing apparatus. The paste from the extrusion apparatus is extruded into the grid mesh, where the paste is dried to form a battery plate of the lead-acid battery. The extruding step can be performed as a sheathing process, a roll-forming process, a tape-casting process, or an injection molding process. For this cureless continuous paste making process to work, basic sulfate crystals with the desired crystal structure must be producible in the extrusion apparatus, and the extruded paste must produce crack-free plates with strong active material cohesion and grid/active material adhesion. Other cureless paste processes for lead acid batteries emphasized using lead sulfate as a starting material. This method, however, resulted in poor active material cohesion and poor grid/active material interface adhesion for the cureless plates. Moreover, formation efficiency and electrical performance of the batteries were poor. Finally, lead sulfate as a starting material increases cost.
There is thus a need to develop a method to produce a paste with the desired crystal structures from a relatively low cost starting material. There is further a need to bind the crystals such that the paste mixture exhibits the desired rheological properties for pasting onto the battery grids to produce crack-free plates with strong cohesion and adhesion. Such method should eliminate steaming and curing steps that significantly add to the cost and time of the battery production.
The present invention provides a plate making process for a lead acid battery which eliminates the need for steaming and curing steps to produce the active material. To this end, and in accordance with the present invention, a paste having a desired crystal morphology is produced in a closed reactor by mixing and reacting an oxidized lead powder, water and sulfuric acid under controlled temperature and mixing conditions. A polymer is then added to the paste to bind the crystals together and to produce desired rheological properties in the paste. In one example of the present invention, a surfactant may also be added to the paste. The paste having the polymer addition is then pasted onto a grid where the paste is dried to form a battery plate of the lead acid battery. This process may be used to form both the positive and negative plates for a lead acid battery. The process produces crack-free plates with strong cohesion and adhesion, high performance and long cycling lives. Moreover, batteries incorporating the plates of the present invention exhibit good electrical performance and durability by using an active material with optimized crystal morphology.